Biosolids (aka treated municipal sewage sludge), the residue produced from waste water treatment process, are produced at a rate of about 5.6 million dry tons per year (ton=2000 lb) in the United States (US) [1]. In general, dry weight per capita production of biosolid resulting from primary, secondary and even tertiary treatment is in average 90 g per person per day [2]. About 61% of the total biosolids currently produced in USA is disposed of through land farming, 17% disposed of in licensed municipal solid waste landfills, 20% incinerated, and about 1% disposed of in surface disposal units. This is by far the largest in volume among the constituents removed by the effluent treatment process; therefore, it's handling methods and disposal techniques are a matter of great concern.
The biosolids produced from the wastewater treatment process is usually in liquid or slurry form. The concentration of solids ranges between 0.25-25% solids by weight. The solid fraction varies between the above limits due to the different methods of the effluent treatment. In the municipal wastewater treatment plant, before disposal, the biosolids has to be treated to eliminate the bacteria, viruses and organic pollutants.
Recently, the sea disposal of sewage sludge has been prohibited, in order to protect the marine environment. The agricultural reuse as a fertilizer, incineration and land-filling has become the principal biosolid disposal method. The latest trends in the field of biosolid utilization are combustion, wet oxidation, pyrolysis, gasification and co-combustion of sewage sludge with other materials for further use as energy source as well as applications in some other areas such as forestry, land reclamation, etc. These applications have generated significant scientific interest. A previous invention of Steam Hydro-gasification technology [3] has been demonstrated to efficiently convert carbonaceous materials into syngas (a mixture of majorly CO and H2). The application of such technology requires external water source and a careful control over the water to carbon ratio inside the feedstocks. Biosolids discharged from waste water treatment plants contains 70% to 97.5% of water based on from which process the biosolids is collected. Traditional disposal of the biosolids waste is costly and environmentally unfriendly. Therefore, a co-utilization of biosolids waste with wasted biomass can be carried out by co-gasification of both feedstocks. The transportation and feed of such feedstocks in pressurized gasification reactor is conventionally done by using dry feed (lock hoppers). While recently, it was widely acknowledged that slurry feed has several advantages over dry feed causing by its nature of operation (Table 1). So a hydro-thermal pretreatment process is invented here to convert the high carbon concentration biomass and biosolids mixture into a slurry formed feedstocks.
TABLE 1Comparison of Feeding Methods in Pressurized Condition4Dry Feed(locker hopper)Slurry feedMethodGas blowPumppressurizationSystemComplexSimpleOperationExpensiveEconomicalDurationUnreliableReliable
The biosolids produced from the wastewater treatment process is usually in liquid or slurry form. The concentration of solids ranges between 0.25-25% solids by weight. The solid fraction varies between the above limits due to the different methods of the effluent treatment. In the municipal wastewater treatment plant, before disposal, the biosolids has to be treated to eliminate the bacteria, viruses and organic pollutants.
There is a need to identify new sources of chemical energy and methods for its conversion into alternative transportation fuels, driven by many concerns including environmental, health, safety issues, and the inevitable future scarcity of petroleum-based fuel supplies. The number of internal combustion engine fueled vehicles worldwide continues to grow, particularly in the midrange of developing countries. The worldwide vehicle population outside the U.S., which mainly uses diesel fuel, is growing faster than inside the U.S. This situation may change as more fuel-efficient vehicles, using hybrid and/or diesel engine technologies, are introduced to reduce both fuel consumption and overall emissions. Since the resources for the production of petroleum-based fuels are being depleted, dependency on petroleum will become a major problem unless non-petroleum alternative fuels, in particular clean-burning synthetic diesel fuels, are developed. Moreover, normal combustion of petroleum-based fuels in conventional engines can cause serious environmental pollution unless strict methods of exhaust emission control are used. A clean burning synthetic diesel fuel can help reduce the emissions from diesel engines.
The production of clean-burning transportation fuels requires either the reformulation of existing petroleum-based fuels or the discovery of new methods for power production or fuel synthesis from unused materials. There are many sources available, derived from either renewable organic or waste carbonaceous materials. Utilizing carbonaceous waste to produce synthetic fuels is an economically viable method since the input feed stock is already considered of little value, discarded as waste, and disposal is often polluting.
Liquid transportation fuels have inherent advantages over gaseous fuels, having higher energy densities than gaseous fuels at the same pressure and temperature. Liquid fuels can be stored at atmospheric or low pressures whereas to achieve liquid fuel energy densities, a gaseous fuel would have to be stored in a tank on a vehicle at high pressures that can be a safety concern in the case of leaks or sudden rupture. The distribution of liquid fuels is much easier than gaseous fuels, using simple pumps and pipelines. The liquid fueling infrastructure of the existing transportation sector ensures easy integration into the existing market of any production of clean-burning synthetic liquid transportation fuels.
The availability of clean-burning liquid transportation fuels is a national priority. Producing synthesis gas (which is a mixture of hydrogen and carbon monoxide) cleanly and efficiently from carbonaceous sources, that can be subjected to a Fischer-Tropsch type process to produce clean and valuable synthetic gasoline and diesel fuels, will benefit both the transportation sector and the health of society. A Fischer-Tropsch type process or reactor, which is defined herein to include respectively a Fischer-Tropsch process or reactor, is any process or reactor that uses synthesis gas to produce a liquid fuel. Similarly, a Fischer-Tropsch type liquid fuel is a fuel produced by such a process or reactor. A Fischer-Tropsch type process allows for the application of current state-of-art engine exhaust after-treatment methods for NOx reduction, removal of toxic particulates present in diesel engine exhaust, and the reduction of normal combustion product pollutants, currently accomplished by catalysts that are poisoned quickly by any sulfur present, as is the case in ordinary stocks of petroleum derived diesel fuel, reducing the catalyst efficiency. Typically, Fischer-Tropsch type liquid fuels, produced from biomass derived synthesis gas, are sulfur-free, aromatic free, and in the case of synthetic diesel fuel have an ultrahigh cetane value.
Biomass material is the most commonly processed carbonaceous waste feed stock used to produce renewable fuels. Biomass feed stocks can be converted to produce electricity, heat, valuable chemicals or fuels. California tops the nation in the use and development of several biomass utilization technologies. For example, in just the Riverside County, California area, it is estimated that about 4000 tons of waste wood are disposed of per day. According to other estimates, over 100,000 tons of biomass per day are dumped into landfills in the Riverside County collection area. This waste comprises about 30% waste paper or cardboard, 40% organic (green and food) waste, and 30% combinations of wood, paper, plastic and metal waste. The carbonaceous components of this waste material have chemical energy that could be used to reduce the need for other energy sources if it can be converted into a clean-burning fuel. These waste sources of carbonaceous material are not the only sources available. While many existing carbonaceous waste materials, such as paper, can be sorted, reused and recycled, for other materials, the waste producer would not need to pay a tipping fee, if the waste were to be delivered directly to a conversion facility. A tipping fee, presently at $30-$35 per ton, is usually charged by the waste management agency to offset disposal costs. Consequently not only can disposal costs be reduced by transporting the waste to a waste-to-synthetic fuels processing plant, but additional waste would be made available because of the lowered cost of disposal.
The burning of wood in a wood stove is a simple example of using biomass to produce heat energy. Unfortunately, open burning of biomass waste to obtain energy and heat is not a clean and efficient method to utilize the calorific value. Today, many new ways of utilizing carbonaceous waste are being discovered. For example, one way is to produce synthetic liquid transportation fuels, and another way is to produce energetic gas for conversion into electricity.
Using fuels from renewable biomass sources can actually decrease the net accumulation of greenhouse gases, such as carbon dioxide, while providing clean, efficient energy for transportation. One of the principal benefits of co-production of synthetic liquid fuels from biomass sources is that it can provide a storable transportation fuel while reducing the effects of greenhouse gases contributing to global warming. In the future, these co-production processes could provide clean-burning fuels for a renewable fuel economy that could be sustained continuously.
A number of processes exist to convert coal and other carbonaceous materials to clean-burning transportation fuels, but they tend to be too expensive to compete on the market with petroleum-based fuels, or they produce volatile fuels, such as methanol and ethanol that have vapor pressure values too high for use in high pollution areas, such as the Southern California air-basin, without legislative exemption from clean air regulations. An example of the latter process is the Hynol Methanol Process, which uses hydro-gasification and steam reformer reactors to synthesize methanol using a co-feed of solid carbonaceous materials and natural gas, and which has a demonstrated carbon conversion efficiency of >85% in bench-scale demonstrations.
Of particular interest to the present invention are processes developed more recently in which a slurry of carbonaceous material is fed into a hydro-gasifier reactor. One such process was developed in our laboratories to produce synthesis gas in which a slurry of particles of carbonaceous material in water, and hydrogen from an internal source, are fed into a hydro-gasification reactor under conditions to generate rich producer gas. This is fed along with steam into a steam pyrolytic reformer under conditions to generate synthesis gas. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/503,435 (published as US 2005/0256212), entitled: “Production Of Synthetic Transportation Fuels From Carbonaceous Material Using Self-Sustained Hydro-Gasification.”
In a further version of the process, using a steam hydro-gasification reactor (SHR) the carbonaceous material is heated simultaneously in the presence of both hydrogen and steam to undergo steam pyrolysis and hydro-gasification in a single step. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/911,348 (published as US 2005/0032920), entitled: “Steam Pyrolysis As A Process to Enhance The Hydro-Gasification of Carbonaceous Material.” The disclosures of U.S. patent application Ser. Nos. 10/503,435 and 10/911,348 are incorporated herein by reference.
All of these processes require the formation of a slurry of biomass that can be fed to the hydro-gasification reactor. To enhance the efficiency of the chemical conversions taking place in these processes, it is desirable to have a low water to carbon ratio, therefore a high energy density, slurry, which also makes the slurry more pumpable. High solids content coal/water slurries have successfully been used in coal gasifiers in the feeding systems of pressurized reactors. A significant difference between coal/water slurries and biomass/water slurries is that coal slurries contain up to 70% solids by weight compared to about 20% solids by weight in biomass slurries. Comparing carbon content, coal slurries contain up to about 50% carbon by weight compared to about 8-10% carbon by weight in biomass slurries. The polymeric structure if cell walls of the biomass mainly consists of cellulose, hemicellulose and lignin. All of these components contain hydroxyl groups. These hydroxyl groups play a key role in the interaction between water and biomass, in which the water molecules are absorbed to form a hydrogen bond. This high hyrgroscopicity of biomass is generally why biomass slurries are not readily produced with a high carbon content.
A number of processes have been developed to produce high carbon content slurries for use as the feedstock for a hydro-gasifier. JGC Corporation in Japan developed the Biomass Slurry Fuel process, which, however must be carried out at semi-critical conditions, with a temperature of 310° C. and at a pressure of 2200 psi. The process converts high water content biomass into an aqueous slurry having a solids content of about 70%, which is the same level as a coal/water slurry. However, it has to be carried out under high energy conditions.
Texaco researchers developed a hydrothermal pretreatment process for municipal sewage sludge that involves heating the slurry to 350° C. followed by a two stage flash evaporation, again requiring high energy conditions.
Traditionally, thermal treatment of wood is a well known technology in the lumber industry to enhance the structural property of wood, but not to prepare a slurry. It decreases hygroscopicity and increases the durability of lumber for construction. Polymeric chains are cleaved in thermal treatment, and accessible hydroxyl groups are reduced leading to a limited interaction with water compared to untreated wood
Aqueous liquifications of biomass samples have been carried out in an autoclave in the reaction temperature range of about 277-377° C. at about 725-2900 psi, to obtain heavy oils rather than slurries, exemplified by the liquification of spruce wood powder at about 377° C. to obtain a 49% liquid yield of heavy oil. See A. Demirba§, “Thermochemical Conversion of Biomass to Liquid Products in the Aqueous Medium”, Energy Sources, 27:1235-1243, 2005.
Our previous work (U.S. patent application Ser. No. 11/489,299) disclosed novel methods that enabled the production of a stable biomass slurry containing up to 60% solids by weight, so as to provide 20-40% carbon by weight in the slurry. However, it was not appreciated at that time the optimal conditions required for using such biomass slurries in hydrogasification processes, such as the optimum viscosity of the slurry to be delivered/pumped.